Stent treatment is a medical technology that has been rapidly progressing in recent years. A stent refers to a mesh-shaped metal pipe that is left indwelling in a body in order to prevent a constricted portion of a blood vessel or the like from narrowing again after being expanded. A stent with a reduced diameter placed in a distal end of a catheter is introduced into the constricted portion, and then subjected to operations of releasing from the catheter and of expanding so as to be placed on an inner wall of a lumen such as the blood vessel. A constricted coronary artery that can cause a myocardial infarction or the like is expanded in a blood vessel expanding operation of inflating a balloon set on the inner wall housing the stent. This type of stent is called a balloon-expandable stent, which uses metal such as stainless alloy or cobalt-chromium alloy.
Among blood vessels connected to the brain, a carotid artery is particularly liable to arteriosclerosis and constriction. Blood clots and plaques accumulated in a constricted portion of a carotid artery flow into the brain to cause cerebral infarction. In this case, a self-expandable stent is used that expands via self-restoration as soon as being released from the catheter. The metal used therefor is a Ti—Ni superelastic alloy that excels in spring characteristics.
Shape-memory alloys including the Ti—Ni alloy are well-known for exhibiting a remarkable shape memory effect associated with a reverse transformation of a martensite transformation. The shape-memory alloys are also well-known for exhibiting a good superelasticity associated with a stress-induced martensite transformation caused by a strong deformation in a parent phase region after the reverse transformation. The superelasticity is remarkably exhibited particularly in the Ti—Ni alloy and in a Ti—Ni—X alloy (X=V, Cr, Co, Nb, or the like) among numerous shape-memory alloys.
The shape memory effect of the Ti—Ni alloy is disclosed, for example, in Patent Document 1. One of the characteristics of the Ti—Ni alloy's superelasticity is that, with the superelasticity starting to act at a reverse transformation start temperature (As temperature) of the alloy, eventually at or higher than a reverse transformation finish temperature (Af temperature) thereof, the alloy deformed by an external constraint restores an original shape thereof as soon as the external constraint is removed, and the shape recovery amount reaches approximately 7% in elongation strain. The As temperature means a shape recovery start temperature, and the Af temperature means a shape recovery finish temperature (shape recovery temperature). A differential scanning calorimeter (DSC) is often used in industry as a measuring device of a transformation temperature. The DSC enables observation of distinct exothermic and endothermic peaks before and after the transformation.
Patent Documents 2 to 4 each disclose an idea of using the Ti—Ni alloy's superelasticity for the self-expandable stent. In tension of the shape-memory alloy in the parent phase at or higher than the Af temperature, stress increases linearly with strain at first. Thereafter, in a consistent manner, the stress-induced martensite transformation occurs as an additional stress is applied, and a loading plateau region of the stress is continued until the strain increases to approximately 7%, and, also when the load is reduced, there is a similar unloading plateau region (refer, for example, to Patent Document 2). The disclosed idea makes use of the superelasticity associated with the distinct transformation. Moreover, in order to obtain a more highly elastic stent, a third element added alloy Ti—Ni—X (X=Nb, Hf, Ta, or W) is disclosed (refer, for example, to Patent Document 2).
The characteristics of the Ti—Ni alloy greatly change with the degree of cold working and heat treatment conditions. The Ti—Ni alloy can be subjected to processing treatment in which the transformation is suppressed so as to provide with a non-plateau superelasticity in which the DSC shows no distinct transformation peaks and the stress increases as the strain increases without exhibiting the plateau region. Such an alloy has been developed as a high-strength Ti—Ni alloy core material that does not have the plateau region for use in guide wires (refer, for example, to Patent Document 5 or 6).